In U.S. Pat. Nos. 3,268,313; 3,421,873 and 3,510,393 by Burgman. et. al, processes and devices were disclosed for forming hollow glass fibers or filaments (hereinafter referred to simply as "fibers").
The usual glass fiber is solid in its cross section and is drawn from a source of molten glass that is passed as a stream through an orifice known as a tip. Upon exiting the tip, the glass assumes the form of a cone and is then cooled and attenuated. In modern processes, as many as 4,000 or more solid fibers may be drawn at the same time from an electrically heated container known as a bushing which has a corresponding number of individual tips thereon. The individual fibers are usually gathered into a single strand which is then wound onto a rotating forming tube. A binder or size is generally applied to the fibers as they are being drawn, but prior to their being gathered, in order to retain them in the configuration of a strand.
Hollow fibers are produced in much the same manner as solid ones except that special tips are employed. As described in the aforementioned patents these tips generally have a hollow tube located concentrically within them through which pressurized gas, usually air, is injected. As the glass is being drawn into a fiber, air exiting from the discharge end of the delivery tube forms a hollow cavity or lumen in the center of the glass stream. As the glass is further attenuated and simultaneously solidified, the lumen is retained.
The hollow delivery tubes utilized in the Burgman, et. al patents were tapered inwardly at an angle of approximately 12.degree. toward their centerline beginning at a point positioned slightly above and inside the terminus of the tip. Satisfactory hollow fibers could be produced if the discharge end of the tube terminated at a distance somewhere between 0.030 and 0.066 inches below the terminus of the tip. Extending the tube beyond the distance of 0.066 inches, as recited in U.S. Pat. Nos. 3,268,313 and 3,510,393, resulted in a situation where the glass necked down rapidly over the tube and fibers having very thin and unstable walls were produced. Attempts to overcome this effect by increasing the pressure of the air used to form the lumen often resulted in rupturing the glass cone. On the other hand, if the discharge end of the tube terminated at a position too close to the exit of the tip, fibers that were intermittently hollow and having relatively thick walls were produced. Careful attention to the amount of air pressure used to create the lumen was also required. If the air pressure became too low, molten glass would tend to wick up inside the tip and cause it to plug. Furthermore, if the tube became displaced from a position other than absolutely parallel and concentric with respect to the centerline of the tip, then eccentric hollow fibers that had extremely large wall thickness variations around their circumference were produced.
Some solutions to the problem of producing hollow fibers having a more uniform wall thickness or concentricity were suggested by the utilization of a special bushing having delivery tubes such as those described in U.S. Pat. Nos. 4,698,082; 4,735,642 and 4,758,259 by Jensen, et al., all of which have been issued to the same assignee as the subject matter of the instant invention. The teachings of the above references show the positioning of counter bored gas discharge tubes at substantially the same elevation as the terminus of the tips. It should be noted that, in this kind of configuration, molten glass exiting from the tip completely engulfs the discharge tube. Although more uniform hollow fibers can be produced using this method, the tube still must be maintained in a perfectly concentric position with respect to the tip and careful attention to the air pressure is still required in order to insure that the glass does not wick back up into the tube or exceed the ability of the surface tension to contain it, thus resulting in a rupture of the glass cone.
Therefore, there still exists a need to develop a method and apparatus for the production of hollow glass fibers which have a high degree of concentricity along with a process less sensitive to some of the problems associated with variations in the pressure used to create the lumen.
The instant invention overcomes some of these problems by using gas delivery tubes that temporarily retard the flow of molten glass through the tip and which extend much farther beyond the terminus of the tip than previously taught in the prior art.